In recent years, a technique for forming a TFT has greatly improved and application development to an active matrix type display device has been advanced. Specifically, a TFT using a polysilicon film for an active layer has higher field effect mobility (also referred to as mobility) than a conventional TFT using an amorphous silicon film, thereby high speed operation is possible. Therefore, a driver circuit formed using a TFT over the same substrate as a substrate over which pixels are formed can control the pixels. In a display device having various circuits formed using a TFT over the same substrate as the substrate over which pixels are formed, various advantages can be obtained such as reducing manufacturing cost, downsizing, rising yield, and reducing throughput.
Research of an active matrix type EL display device having an electroluminescent element which is a light emitting element (hereinafter referred to as an EL element) as a display element in each pixel of a display device has been activated. The EL display device is also called an organic EL display (OELD: Organic EL Display) or an organic light emitting diode (OLED: Organic Light Emitting Diode).
Generally, a current value flowing to an EL element is proportionally related to a light emission luminance of an EL element, thereby in an EL display device using an El element as a display element, luminescence is controlled by the current value. Therefore, in the EL display device, proposed has been a pixel configuration which is different from a liquid crystal display device which controls luminance by a voltage value (see Patent Document 1).
Further, proposed has been a driver circuit for inputting a video signal to each pixel of an EL display device as described in Patent Document 1 (Patent Document 2).    [Patent Document 1]            International Publication No. WO 01/06484 pamphlet            [Patent Document 2]            International Publication No. WO 02/39420 pamphlet        